Preheat to 325F,Beat eggs, butter, sour cream, milk, salt, and basil.,Stir in cheese, mushrooms, onions, and tomato.,Pour into 9x13 dish, cook uncovered until omelet is set but moist - about 40-45 minutes.,After 30 minutes, stir so the ingredients are evenly distributed to the outer parts of the dish (they tend to float to the center).,This can be made 24 hours in advance.,______